Lead (Pb)-containing solders have been used in microelectronic applications to form electrical interconnections between packaging levels, to facilitate heat dissipation from active devices, to provide mechanical/physical support, and to serve as a solderable surface finish layer on printed circuit boards (PCBs) and lead frames. For example, Pb-containing solders are employed in C (controlled collapse chip connection) solder joints, or bumps, connecting a silicon chip to a BT-resin-based laminate chip carrier and BGA solder joints, or balls, connecting the module to a PCB. The C, or flip-chip, solder joints are generally used to attach flip-chips to ceramic multichip modules (MCMs) for mainframe computers or high-end servers. The flip-chip has become pervasive because it provides a high-I/O-count, high-density interconnection scheme with proven performance and reliability. With respect to Pb—Sn packaging technology, the C solder joints used in high-performance systems are usually fabricated from Pb-rich solders such as 95Pb-5Sn or 97Pb-3Sn, while the flip-chip joints used in consumer products are fabricated from the eutectic Sn—Pb alloy, 63Sn-37Pb. In some instances, chips with 97Pb-3Sn C4 bumps are attached to the module using the 63Sn-37Pb alloy. Since C4 or flip-chip solder joints provide an area array connection, they yield the highest chip interconnection density compared with other interconnection schemes, such as wire bonding or tape automated bonding (TAB), which provides mainly peripheral interconnections to a chip.
The combined solder bump structure of 97Pb-3Sn on the side of a semiconductor chip and 37Pb-63Sn on the side of organic substrate has been used for several years in microprocessor packaging. The use of such high-melting alloys for chips provides a hierarchy in soldering temperatures so that the 97Pb-3Sn solders (melting temperature 315° C.) on the side of the chip do not melt and the under bump metallurgy is not consumed during the next level flip chip assembly using low melting solders such as eutectic 37Pb-63Sn solders with a melting temperature of 183° C.
The use of Pb-free solders is increasing in the electronic industry. In general, the Pb-free solder alloys have a higher melting temperature as compared to eutectic SnPb, so near eutectic composition of Pb-free solders such as Sn0.7 wt % Cu, Sn3.5 wt % Ag, or SnAgCu alloys (melting temperatures around 217-221° C.) are available as Pb-free solder bumps for the flip chip technology on organic substrates because they have a single and low melting point compared to the other Pb-free solder alloys. However, the high tin (Sn) content of these common eutectic Pb-free solder bumps are typically unreliable and subject to failure due to the rapid consumption of UBM (Under Bump Metallurgy) during several reflow process and during the operation at high temperature under high current. Therefore, a two-layered structure with high/low solder temperature hierarchy, as replacement of the 97Pb—Sn/37Pb—Sn structure, needs to be developed to permit effective multi-chip module assembly and reliable under high current stressing.
Eutectic Au20 wt % Sn solder has the advantages of smaller Sn content and higher melting point, 280° C. It reduces the consumption of UBM during chip join and subsequent multiple reflow processes and has no fatigue issues. However, its high cost is prohibitive for high volume commercial product applications. The cost of eutectic AuSn solder is 500 times higher than Sn0.7 wt % Cu or Sn3.5 wt % Ag when considering the same volume. Further, a second problem with an AuSn solder is that the exact eutectic composition needs to be maintained throughout the process to avoid the abrupt increase of the melting temperature of Au—Sn solder.
As the trend in the electronics industry continues to push towards high performance and miniaturization, smaller solder bump interconnections at fine pitch are in great demand for flip chip technologies. When the diameter of solder bumps decreases for a finer pitch, the stand off height between a chip and its substrate also decreases. The low stand-off height is a concern from the fatigue life point of view due to higher shear strain after assembly. Also, the low stand-off height between the chip and the substrate and the small gap among the joints makes it difficult to clean the flux residue and underfill the module. Therefore, it is desirable to form flip chip solder joints with a larger height and a larger gap between the joints.
U.S. Pat. No. 6,893,799 B2, discloses a combination of two known solder bumping technologies, electroplating and injection molded solder (IMS) wherein a small layer of solder (Sn) or metal (Au, Ag, and Pd) is deposited by an electroplating method and the required solder volume is deposited by IMS. The first solder is deposited by electroplating method so as to serve as a wettable layer for the second solder which is deposited by IMS. Further, the electro plated solder structure (1st solder) mix with bulk solder (2nd solder) is subsequently deposited by IMS because the melting temperature of the 1st electrodeposited solder is not higher than that of the 2nd IMS, so the final composition of solder bump on the wafer is the mixed composition of the two alloys. Also, the electroplating method of the first solder can not handle wide alloy range.
U.S. Pat. No. 6,348,401 B1 describes a two-step solder bump fabrication process, including a first step of electroplating solder and a second step of screen-printing solder paste over the electroplated solder layer. The fabricating step is a solder-reflow process to reflow each combined structure of electroplated solder layer and printed solder layer into one single solder bump. This method is very expensive in that a thick photoresist process is needed to fabricate two step of electroplating and screen printing method.
U.S. Pat. No. 6,281,041 B1 discloses a method for forming a solder interconnection structure which has an annular solder non-wetting copper oxide layer between two solder materials having different melting points. The process includes the deposition of a blanket copper layer, photoresist lift off, and copper oxidation process to form the annular solder non-wettable copper oxide layer over the upper dome portion of the first solder bump which has a higher melting point than a second solder material. This method is very complicated and very expensive because it needs several extra process steps compared to the common solder bumping process.
U.S. Pat. No. 2007/0059548 A1 discloses an attachment system with a high-liquidus alloy column, an intermediate-liquidus solder, and a low-liquidus solder. The column of high-liquidus alloy is cast or cut from wire or ribbon and does not provide a manufacturable method to provide alignment between high-liquidus alloy column and low-liquidus solder
There are various techniques that can be used to implement a fine pitch solder bumping process, including, for example, evaporation, electroplating, stencil printing, ball drop, and so on. All of the above mentioned techniques are possible to deposit two steps solders of different composition. However, evaporation and electroplating method can not handle wide alloy range and the reflow of two solders should be done together at high temperature. Further, the resulting composition will definitely be mixed and the side wall of UBM should be conformably covered with the first solder. Stencil printing and ball drop method could be applied for the deposition of multi-component solder alloys, however, in the case of stencil printing method, there is practical limitation on the extendibility of it for small bump size and fine pitch below 75 um on 150 um, respectively because the solder pastes are composed of a mixture of solder metal powder and flux. The ball drop method may also impose limitation of bumping of very small volume of the first solder which is used for the layer passivating the UBM.
A need exists for a two-layered solder bump fabrication method including a Pb-free, high/low or low/high temperature, solder hierarchy pair while still exhibiting effective multi-chip module assembly and reliability under high current stressing.
IA further need exists for reducing the consumption of UBM due to the formation of intermetallic compounds during multi-chip module assembly or high current stressing. A need exists for a greater stand-off height between the semiconductor chip and the substrates, to facilitate the underfill process and improves fatigue resistance due to the taller bump. A structure is needed having limited wafer damage from corrosion or contamination, less void formation inside the bumps, a larger number of interconnections with finer pitch than screen printing methods and elimination of the volume reduction problem of solder paste. Further, a need exists for more flexibility in selecting the composition and multi-component solder alloys when compared with the conventional electroplating method. In addition, a need exists for a simple IMS process and an easily manufacturable method to provide alignment between high melting temperature solder and low melting temperature solder.
Other and further objects, advantages and features of the present invention will be understood by reference to the following specification in conjunction with the annexed drawings, wherein like parts have been given like numbers.